Plasmacytoid Dendritic Cells (pDC) specialize in secreting high levels of type I interferons (IFNs) and proinflammatory chemokines in response to certain viruses. They also stimulate T cell proliferation and differentiation in vitro. Consequently, pDC are viewed as critical in protecting the host from incoming pathogens. This proposal focuses on the role of pDC in anti-tumor immune responses. In our preliminary studies, we analyzed a panel of MCA-induced sarcomas including tumors that are rejected by the immune system in wild type mice (immunogenic tumors) and tumors that grow progressively (non immunogenic tumors). Results demonstrate that: (a) effective rejection of immunogenic sarcomas requires the action of type 1 IFN s on host cells; (b) pDC infiltrate immunogenic MCA-induced sarcomas but not non-immunogenic tumors. Therefore, we hypothesize that pDC infiltrating immunogenic tumors may provide a major source of type I IFNs and proinflammatory chemokines that activate host anti-tumor immune responses. We are uniquely positioned to test this hypothesis because we can analyze the growth of sarcomas following elimination or functional paralysis of pDC by antibodies that specifically deplete or block pDC function in vivo. Our preliminary experiments also demonstrate that pDC infiltrate human melanomas, but appear to be functionally inactive, possibly due to immunosuppressive factors in the tumor microenvironment. Can we reactivate pDC within tumors? Because they express toll like receptor (TLR) 9 and TLR7, pDC can be activated in vitro and in vivo by CpG oligonucleotides and imidazoquinolins. Our preliminary findings demonstrate that CpG-stimulated pDC effectively delay growth of murine B16 melanomas. In addition, we have observed that CpG and one TLR7 ligand, resiquimod, differ in their ability to elicit type I IFN responses in pDC, yet trigger similar secretion of proinflammatory cytokines. Given this, we will test and compare efficacy of CpG- and resiquimod-activated pDC for immunotherapy of poorly immunogenic tumors, including: a) B16 metastatic melanoma; b) a mouse model of spontaneous breast carcinoma expressing a defined tumor-associated antigen, which will also allow us to study specific interactions between pDC, T cells and dendritic cells. Our preliminary findings validate the concept that activated pDC at the tumor site initiate and amplify anti-tumor immune responses. These findings also substantiate the physiological relevance of the proposed studies to clinical human cancer situations.